Project Summary Epidermodysplasia verruciformis (EV) was the first described primary immunodeficiency (PID). By 1946, it was shown by Lutz to be an autosomal recessive (AR) predisposition to skin-tropic viruses, prior to the description of congenital neutropenia by Kostmann (1950). Its lack of associated immunological phenotypes long prevented its recognition as a PID. EV is characterized by disseminated and persistent flat warts, which often evolve into skin cancer. The lesions are caused by E5- and E8-deficient members of the ? genus of human papillomaviruses (HPVs), which exclusively reside in keratinocytes and remain silent in the general population. EV typically strikes otherwise healthy individuals (?isolated EV?), or rarely occurs in the context of other infectious diseases (?syndromic EV?). In 2002, bi-allelic mutations in TMC6 and TMC8, encoding EVER1 and EVER2, were found in patients with isolated EV, whose T cells were normal. Bi-allelic mutations in CIB1 were reported in 2018 in other patients with isolated EV. Remarkably, CIB1, EVER1, and EVER2 form a complex that binds to E5 and E8. This complex operates as a restriction factor governing keratinocyte-intrinsic immunity to ?-HPVs. From 2012 onward, mutations in RHOH, STK4, and other T cell genes were found in patients with syndromic EV. We hypothesize that other, related single-gene inborn errors of cutaneous immunity against ?-HPVs, underlie EV in other patients. The goal of this application is thus to analyze in greater depth the molecular and cellular basis of isolated and syndromic EV. First, we will discover new genetic etiologies of EV thanks to the ongoing recruitment of unrelated EV families, by combining genome-wide linkage (GWL) and whole exome sequencing (WES). Second, we will functionally characterize the novel genotypes by studying the mutant proteins in isolation and in the patients? cells, including their relationship with the products of the known EV-causing genes, such as the EVER-CIB1 complex in keratinocytes and RhoH or STK4 in T cells. Third, we will model HPV infection of keratinocytes in the presence of T cells in vitro, with viral proteins and particles, using keratinocyte cell lines, foreskin keratinocytes, the patients? keratinocytes, or induced pluripotent step cell (iPSC)-derived keratinocytes, which will be edited by CRISPR/Cas9. Our project is highly innovative yet supported by strong preliminary data. Indeed, we have recruited 52 novel families, identified three novel genetic etiologies, underlying isolated (mutations in RBPJ) or syndromic EV (ITGAL and OX40), and began elucidating their mechanistic connection with keratinocytes and T cells, respectively. Our research provides novel insights into the mechanisms of cutaneous immunity to ?-HPVs, highlighting the dual contribution of keratinocyte-intrinsic immunity and T-cell adaptive immunity. Our research benefits EV patients and families, with the development of novel diagnostic approaches, including genetic counseling, and facilitating the development of novel therapeutic approaches based on a rational understanding of the pathogenesis. Finally, the study of EV is a fruitful model to analyze other mucosal and cutaneous illnesses caused by other HPVs.